1. Field of the Invention
The present invention relates to a fuel cell humidifying system having a water permeable humidifier, and relates in particular to a technology for preventing freezing of humidifier and resulting clogging of the fuel cell.
2. Description of the Related Art
A fuel cell that uses a solid polymer membrane for the electrolyte membrane, for example, is comprised by stacking a plurality of unit cells.
A unit cell is comprised by a core of a solid polymer membrane of ionic conductivity in intimate contact with a fuel-electrode (anode) and an air-electrode (cathode) on each surfaces thereof. When hydrogen gas is supplied through the fuel gas passage formed as an U-shaped groove on the surface facing the fuel-electrode and air is supplied simultaneously through the oxidizer passage formed as an U-shaped groove on the surface facing the air-electrode, electricity is generated as a result of electrochemical reaction between the electrodes in each unit cell.
To maintain high electrical generation efficiency, the solid polymer membrane must be maintained in a water-saturated state so as to enable the solid polymer to function as a proton (hydrogen ions) conductive electrolyte.
However, during the electricity generation process, the solid polymer membrane is sometimes subjected to conditions that promote drying, due to various factors including the fact that the water generated by chemical reaction is transported out of the system. Therefore, to maintain good ionic conductivity, it is necessary to supply moisture to the solid polymer membrane.
For this reason, a conventional approach is to devise a humidifying system to humidify dry gases such as the air to be supplied to the air-electrode side and the hydrogen to be supplied to the fuel-electrode side by passing both gases through a water permeable humidifier so as to obtain wet gases for humidifying the solid polymer membrane.
Here, the term xe2x80x9cdry gas(es)xe2x80x9d means the gas has a relative low humidity, such as the gas contains little moisture or contains moisture insufficient to humidify the solid polymer membrane, and the term xe2x80x9cwet gas(es)xe2x80x9d means the gas has a relative high humidity, such as the gas contains moisture sufficient to humidify the solid polymer membrane.
This type of humidifier is provided with a hollow threaded membrane to permit water to infiltrate in the direction of the film thickness (refer to Japanese Patent Application, First Publication, No. Hei 7-71795, and Japanese Patent Application, First Publication, No. Hei 8-273687), and for example, humidification of the air-electrode side is carried out as follows.
That is, while dry air is being forced through a jacket containing a packing comprised by an assembly of hollow thread membranes using a charging apparatus such as a supercharger, wet out-gas discharged from the air-electrode side is forced through the hollow section inside the hollow thread membrane so that the moisture contained in the wet out-gas can infiltrate through the porous surface of the hollow thread membrane and disperse on the outside of the hollow thread membrane as water vapor to add humidity to the dry air flowing through the inter-thread spaces formed by the thread membranes.
However, in the fuel cell humidifying system provided with a water permeable humidifier, water is produced by the redox reaction of oxygen and hydrogen, and the produced water is recovered from the wet out-gas and transferred to the dry gas through the hollow thread membrane in the humidifier, and is reused to add humidity to the fuel cell. Therefore, during the cold spell or in cold climate, it is unavoidable to subject the moisture in the humidifier to freezing conditions.
Freezing creates problems in the following manner. Because the pore diameter of the hollow thread membrane is extremely fine (4 nm) and can exert a very high surface tension force, the moisture condensed in the pores is subjected to supercooling and does not undergo phase transformation (water to ice) but, because the internal diameter of the hollow section of the hollow thread membrane is relatively large (0.37 mm) and is not subjected to supercooling, the moisture becomes frozen.
When the moisture becomes frozen in the interior of the hollow thread membrane, it becomes difficult to operate the system at its optimal capacity.
Also, it is also possible that, if dust particles and the like having diameters larger than the inside diameters of the hollow section or the pores of the membrane are mixed in the wet out-gas from the fuel cell, such particles may block the entrance to the hollow section or plug up the pores of the membrane to cause a pressure increase at the entrance to the hollow thread membrane or decrease in the recoverable amount (percent) of water, resulting in degradation in the performance of the inherent capability of the fuel cell humidifying system.
Similarly, for the dry gas supplied by the supercharger and the like through the gas passage may also be subjected to detrimental effects of freezing and clogging.
Accordingly, when freezing or clogging is generated in the hollow thread membrane of the humidifier, it becomes difficult to operate the fuel cell at its inherent optimal capability by continuing to recover a sufficient amount of water from the wet out-gas to resupply an appropriate degree of humidity to the fuel cell.
The present invention is provided in view of the background information described above, and it is an object of the present invention to provide a technology for preventing freezing and clogging in the humidifier.
To resolve the problem described above, the present invention provides the following structure of the fuel cell humidifying system.
That is, the fuel cell humidifying system of the present invention is for providing a dry gas (in the embodied case, dry air Ad) and an exhaust gas (in the embodied case, wet out-air OAw) discharged from the fuel cell (1) into a water permeable type humidifier (3) so as to recover moisture contained in the exhaust gas in the dry gas to produce a wet gas (in the embodied case, wet air Aw) which is supplied to the fuel cell; and the system has a flow path switching mechanism (in the first embodiment, this mechanism comprises a first three-way valve 4, a second three-way valve 5, a flow adjusting valve 6, sweep piping 9, and a control apparatus; and in the second embodiment, this mechanism comprises first reverse cleansing piping 31, second reverse cleansing piping 32, first exhaust piping 33, second exhaust piping 34, a pressure sensor 35, a first shutoff valve 41, a second shutoff valve 42, a third shutoff valve 43, a fourth shutoff valve 44, a fifth shutoff valve 45, a sixth shutoff valve 46, a seventh shutoff valve 47, and a control apparatus) for switching gas passages (in the embodied case, air piping 7, out-air piping 8) leading to the humidifier.
According to the fuel cell humidifying system, clogging in the humidifier can be prevented by switching the flow direction in the humidifier.
In this case, the flow path switching mechanism may adopt a mechanism for flowing a dry gas in the exhaust gas passage in the humidifier (in the embodied case, the wet out-air passage) or a mechanism for reversing the flow direction of exhaust gas in the humidifier.
In the former flow path switching mechanism, by flowing the dry gas in the exhaust gas passage in the humidifier before the moisture freezes, water vapor that causes freezing can be swept out so that freezing can be prevented beforehand.
In the latter flow path switching mechanism, even if clogging are generated in the exhaust gas passage in the humidifier by dust particles and other such substances contained in the exhaust gas flowing in the forward direction, because the exhaust gas flow direction can be reversed, such clogging can be eliminated by reverse flow cleansing.